1. Field of the Invention
The present invention relates to a method and an apparatus for the rapid thermal processing (RTP) of sensitive electronic materials. The present invention allows high throughput of wafers which must be rapidly processed inside a small enclosure to reduce the loss of material from the wafer surface.
2. Description of the Prior Art
Rapid Thermal Processing (RTP) is a versatile optical heating method which can be used for semiconductor processing as well as a general, well controlled, method for heating objects or wafers which are in the form of thin sheets, slabs, or disks. The objects are generally inserted into a chamber which has at least some portions of the chamber walls transparent to transmit radiation from powerful heating lamps. The transparent portion of the walls is generally quartz, which will transmit radiation up to a wavelength of 3 to 4 microns. These lamps are generally tungsten-halogen lamps, but arc lamps or any other source of visible and/or near infra-red radiation may be used. The radiation from the lamps is directed through the transparent portions of the walls on to the surface of the object to be heated. As long as the objects absorb light in the near infrared or visible spectral region transmitted by the transparent portion of the walls, RTP techniques allow fast changes in the temperature and process gas for the different material processes and conditions. RTP allows the "thermal budgets" of the various semiconductor processes to be reduced, as well as allows the production of various metastable states which can be "frozen in" when the material is cooled rapidly.
RTP systems are relatively new. In the last 10 or 15 years, such systems were used only in research and development. The thrust of the work was increasing the temperature uniformity, and developing heating cycles and processes which decreased the thermal budget. Prior art RTP machines can heat unstructured, homogeneous materials in the form of a flat plate or disk, and produce temperature uniformities across the plate adequate for semiconductor processing processes.
The temperature control in current RTP systems is mostly performed by monochromatic (or narrow wavelength band) pyrometry measuring temperature of the relatively unstructured and featureless backside of semiconductor wafers. The results of the temperature measurement are generally used in a feedback control to control the heating lamp power. Backside coated wafers with varying emissivity can not be used in this way, however, and the backside layers are normally etched away or the temperature is measured using contact thermocouples.
A newer method of temperature control is the power controlled open loop heating described in U.S. Pat. No. 5,359,693, which patent is hereby incorporated by reference.
German patent DE42 23 133 C2, hereby incorporated by reference, discloses a method of producing relatively defect free material in RTP machines. Apparatus induced thermal inhomogeneities have been reduced in the last few years because of the demand for more uniform processing. Among the techniques used have been control of the individual lamp power, use of circular lamps, and rotation of the semiconductor wafers with independent power control.
Most RTP machines have a thin rectangular quartz reaction chamber having one end open. Chambers meant for vacuum use often have a flattened oval cross section. Chambers could even be made in the form of a flat cylindrical pancake. In general, the chambers are used so that the thin objects to be heated are held horizontally, but they could also be held vertical or in any convenient orientation. The reactor chamber is usually thin to bring the lamps close to the object to be heated. The reactor chamber is opened and closed at one end with a pneumatically operated door when the wafer handling system is in operation. The door is usually made of stainless steel, and may have a quartz plate attached to the inside. The process gas is introduced into the chamber on the side opposite the door and exhausted on the door side. The process gas flow is controlled by computer controlled valves connected to various manifolds in a manner well known in the art.
Reactors based on this principle often have the entire cross section of one end of the reactor chamber open during the wafer handling process. This construction has been established because the various wafer holders, guard rings, and gas distribution plates, which have significantly greater dimensions and may be thicker than the wafers, must also be introduced into the chamber and must be easily and quickly changed when the process is changed or when different wafer sizes, for example, are used. The reaction chamber dimensions are designed with these ancillary pieces in mind. Copending patent application Ser. No. 08/387,220, now U.S. Pat. No. 5,580,830, assigned to the assignee of the present invention, hereby incorporated by reference, teaches the importance of the gas flow and the use of an aperture in the door to regulate gas flow and control impurities in the process chamber.
The wafer to be heated in a conventional RTP system typically rests on a plurality of quartz pins which hold the wafer accurately parallel to the reflector walls of the system. Prior art systems have rested the wafer on an instrumented susceptor, typically a uniform silicon wafer. Copending patent application Ser. No. 08/537,409, assigned to the assignee of the present invention, hereby incorporated by reference, teaches the importance of susceptor plates separated from the wafer.
Rapid thermal processing of II-VI and III-V semiconductors has not been as successful as RTP of silicon. One reason for this is that the surface has a relatively high vapor pressure of, for example, arsenic (As) in the case of gallium arsenide (GaAs). The surface region becomes depleted of As, and the material quality suffers.
Kanack et al., Appl. Phys. Lett. 55, 2325, (1989) disclose a method of annealing contacts in GaInAsP by placing the InP substrate wafer between two silicon susceptor wafers.
Katz et al., J. of Vac. Science and Tech. B 8, 1285, (1990); Pearton et al. in SPIE 1393, 150, (1991), and Kazior et al. IEEE transactions on Semiconductor Manufacturing 4, 21 (1991) teach a method of enclosing a compound semiconductor wafer in an enclosed susceptor having an enclosed volume only slightly greater than the wafer volume, then placing the susceptor in the RTP system for processing. A sacrificial wafer is processed first to charge the walls of the interior of the enclosed susceptor with the volatile component of the wafer of interest, and thereafter a number of product wafers can be treated. Presumably, the partial pressure of the volatile component inside the susceptor is high enough that the rate of evaporation from the wafer surface is equal to the rate at which the volatile component redeposits on the wafer surface.
Such a process does not lead to high throughput, however. The wafer and the enclosing susceptor must be flushed for a relatively long time to expel all the oxygen which was introduced into the enclosure when the wafer was introduced.
The enclosed susceptor could be flushed relatively rapidly if the vacuum RTP system is used. However, these systems have much greater cost and complexity.